UvA-DARE (Digital Academic Repository) The role of OTX2 in medulloblastoma Bunt, J. Publication date 2012 Link to publication Citation for published version (APA): Bunt, J. (2012). The role of OTX2 in medulloblastoma. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:27 Sep 2021 1 General introduction Medulloblastoma as a heterogeneous disease Histology and clinical aspects of medulloblastoma 1 Medulloblastoma is a malignant embryonal tumor arising from the cerebellum (Figure 1). With an incidence just below 1 per 100,000 children, it is the most common malignant brain tumor in children [1, 2]. Boys have a higher incidence than girls (7.5 and 4.8 per 1,000,000 boys and girls respectively) [3] and the incidence peaks at 7 years of age [4]. However, tumors also occur in adults, albeit far less frequent than in children [5-7] Figure 1. Sagittal (A) and axial (B) section of a MRI scan of a patient with a medulloblastoma. Adapted from Figure 1 of Crawford et al. 2007 [3]. The 5-year overall survival rate of medulloblastoma patients has increased over the past decades from 30% [8] to 60%-80% [3, 9, 10]. This is largely a result of improved treatment strategies, which currently consist of surgical resections, radiation therapy and chemotherapy. However, many children still die from this disease or suffer from long-term side effects of the treatment they receive [11-15]. In the current risk stratification, patients are stratified as average risk if the patient is older than 3 years, has no metastatic disease and has a near-total resection (<1.5 cm of residual disease). Both metastatic disease and residual disease have been shown to decrease outcome [16]. Younger children are classified as high risk, because no radiotherapy is performed in that age group. This risk stratification is however limited in its prediction of the outcome for the patients. Based on the histology of the tumor, the pathological classification of the World Health Organization recognizes classical medulloblastoma and 4 variants thereof [5]. Classical medulloblastomas represents the majority of tumors (60 – 80%). These tumors are characterized by a sheet of small, round, densely packed cells with hyperchromatic nuclei and limited cytoplasm [5]. The nodular/desmoplasmic medulloblastomas, which are the most common of the variants, are defined by dense nodules of differentiated neurocyctic cells 11 surrounded by high proliferating undifferentiated cells with intercellular reticulin [5]. When these tumors mainly consist of these nodules, the tumor is classified as a medulloblastoma with extensive nodularity. Large cell medulloblastomas are characterized by large cells with prominent nucleoli and more cytoplasm compared to classic medulloblastomas. The last variant, anaplastic medulloblastoma, shows enlarged, tightly packed pleomorphic nuclei showing angulation, mounding and wrapping. In light of the clinical outcome, the tumors with either large cell or anaplastic histology are associated with the worst outcome, while patients with desmoplastic/nodular medulloblastomas have a slightly improved outcome [17, 18]. However, the clinical significance is limited and precise classification is difficult, as multiple features can be found within one tumor. Although the overall survival of medulloblastoma patients has increased, there is still much to gain from better risk stratification and improved treatment. Towards that end, the biology of the tumor needs to be better understood to recognize the molecular events that are at the genesis of the tumor. Biology of medulloblastoma The first clues of the molecular pathways potentially involved in medulloblastoma tumorigenesis came from familial cases. In cancer predisposition syndromes, including Gorlin, Turcot, Li-Fraumeni, blue rubber-bleb nevus, Nijmegen Breakage and Rubinstein- Taybi syndrome, patients have an increased risk to develop medulloblastoma [19, 20] [21-26]. As the molecular events in these tumors have been characterized, their role in medulloblastoma has also been investigated. Germline mutations in PTCH1 are the most common cause of Gorlin syndrome. Patients mostly develop basal cell carcinoma, but also have an increased risk for developing medulloblastoma [27]. The mutations in PTCH1 result in an aberrant activation of the sonic hedgehog (SHH) pathway, which plays a critical role in normal development of the cerebellum. Besides somatic PTCH1 mutations, mutations in other components of the SHH pathway, including PTCH2, SMO and SUFU, which all cause SHH pathway activation, also occur in sporadic medulloblastoma [28-33]. A range of mouse models for medulloblastoma have been developed based on activated SHH signaling [34]. Patients with Turcot syndrome caused by APC mutations also have an increased risk of developing medulloblastoma. When the tumor suppressor APC is mutated, it is unable to halt the activity of the wingless (WNT) pathway, causing aberrant activation of this pathway. In sporadic medulloblastoma APC mutations are rare [35]. However, in 2-10% of all medulloblastoma, activating mutations are found in CTNNB1 (beta-catenin), which is 12 a downstream effector of the WNT pathway [35-41]. These mutations enable CTNNB1 to escape degradation, to localize in the nucleus and to function as a transcriptional activator of genes, including MYC and CCND1. Other aberrations within the WNT pathway, affecting AXIN1 or AXIN2, have also been reported in medulloblastoma [38, 42]. Recently, the first 1 mouse model for medulloblastoma with an aberrant WNT signaling has been described [43]. The frequency of mutations ofTP53 , which causes Li Fraumeni syndrome, has been variable within medulloblastoma literature, ranging from 0 to 10% [44-48]. Mutations are mostly found in metastatic disease, aggressive tumors or recurrences after treatment [48, 49]. Interestingly, this poor prognosis has not been validated in other studies, where somatic P53 mutations commonly co-occur with CTNNB1 mutations in tumors, thereby predicting a favorable outcome for the patients [46, 50]. In contrast, however, medulloblastomas with germline P53 mutations always show activation of SHH signaling and are characterized by massive chromosome rearrangements which occur in a one-step event called chromothripsis [51] Often these DNA rearrangements lead to high-level amplifications of genes activating the SHH pathway. Although the overall frequency of P53 mutations in human medulloblastoma is low, many mouse models of medulloblastoma are derived in p53 null background [34]. Cytogenetic analyses of medulloblastoma also provided candidates for medulloblastoma biology. The most common genetic aberration in medulloblastoma is loss of chromosome arm 17p. This is often coinciding with a gain of chromosome arm 17q in the form of an isochromosome 17q [52-57]. Concomitant with these gains, chromosome arm 17p is lost. Many genes in this region have been proposed to play a role in medulloblastoma, such as TP53, REN, MNT and HIC. However, it is unclear how the reduced expression of these genes contributes to medulloblastoma formation, as the other allele of these genes seems mostly unaffected [48]. Also, a second hit of the remaining 17p has not been identified. A possible explanation could be haploinsufficiency of genes on 17p. Alternatively, a mild increased expression of genes on 17q might be beneficial for the tumor, as i17q seems to occur more often in recurrent tumors than in primary tumors [58]. Other major cytogenetic aberrations include monosomy of chromosome 6, loss of 9q and gains of 1q and 7 [1, 54, 55, 59-61] Cytogenetic analyses also commonly find amplifications of members of the MYC family. MYC and MYCN amplification are most frequent (3-10% each) [10, 54, 55, 62-64], but MYCL amplifications also occur [65, 66]. Other amplifications have been found in medulloblastoma as well, but their frequencies are much lower. Examples include OTX2 ([67-70], ERBB1 and ERBB2 [71-73], TERT [74], GLI1 and GLI2 [75], cell cycle genes such as MDM2 [71], CDK4, CDK6 [65], CCND1, CCNE2 [65] and histone modifying genes such as JMJD2B, JMJD2C and 13 MYST3 [65, 76]. Homozygous deletions have been described for CDKN2A (p16) [77] and several histone modifying genes such as DMBT1, EHMT1, SMYD4, L3MBTL3, SCML2 [65, 78] Finally, a number of genes and pathways have been investigated for (in)activating mutations. However, only sporadic mutations have been reported for NRAS [79], CXCR4 [80], P75 [81], PIK3CA [82], PMS2 [83], BRCA2 [84], MSH6 [85], NBS [86-88] and MXI [89]. Recently, the first genome-wide screening for mutations in medulloblastoma has been published. Besides mutations in genes such as PTCH1 and CTNNB,